Abstract—Investigations concerning the occurrence of disulphide interchange for disulphides related to cystine are made with either a single unsymmetrical disulphide or an equimolar mixture of two symmetrical disulphides. It is found that in the pH range 1–6 where spontaneous interchange can usually be disregarded, interchange can be initiated by the addition of a small amount of a thiol or by irradiation with ultraviolet light from either of two artificial sources, interchange being more rapid at pH 4–6 than at pH 1–3. Direct sunlight and, to a lesser extent, visible (filtered tungsten) light are also effective. Disulphide interchange in the solid state can also be promoted by ultraviolet light. A comparison is made between the roles' of S‐S and C‐S bond fission in the photochemistry of disulphides, with respect not only to the promotion of disulphide interchange, but to other phenomena such as the dismutation of disulphides to thioethers and t